Apparatus for rendering audio signals audible usually include a device for volume control. However, the perception of loudness of the human ear varies with frequency. In case of low sound pressures, low frequencies are felt to be less loud than high frequencies. This can be depicted in the form of a subjective perception intensity scale, for which also the term loudness is used. In the present paper, this will also be referred to as aurally compensated loudness.
With higher-quality audio equipment, for instance stereo sets or stereo car radios, this subjective perception intensity scale is taken into consideration by performing a physiologically matched loudness control.
A conventional physiologically matched, i.e., aurally compensated, loudness control which is shown in the book Phono Technik by P. Zastrow, Frankfurter Fachverlag, 1984, pages 141-144, and from which claim 1 starts, uses for loudness control a potentiometer connected between an audio signal source and ground. The potentiometer has a slidable tap and a fixed tap. The attenuated audio signal is taken off from the slidable tap. Between fixed tap and ground there is connected a capacitor serving for paying regard to auditory sensation. As the frequency of the audio signal increases, the resistance distance or path of the potentiometer between fixed tap and ground is increasingly bridged by the capacitor. While the position of the slidable tap remains the same, lower frequencies of the audio signal are thus attenuated to a lesser extent than high frequencies of the audio signal, i.e., lower frequencies are emphasized as compared to high frequencies.
Digitalization is increasingly gaining ground in audio equipment of the type mentioned. Desirable are devices rendering possible digital loudness control of an analog audio signal.